If the high-T.sub.c superconductors are to be used in practical power applications, either for power transmission or for magnetic windings, they will need to be drawn as wire or grown as films on flexible substrates, which will probably be wound on metal support cables or magnetic yokes. To date, three approaches have achieved significant results: the extrusion of bismuth-based superconductors, melt-textured growth of bulk YBa.sub.2 Cu.sub.3 O.sub.7-x (YBCO), and growth of oriented thin films on flexible substrates.
Many groups have reported successful epitaxial growth of high quality thin films on singly crystalline substrates. High quality is ultimately defined by high values of the superconducting transition temperature T.sub.c and the critical current J.sub.c defining the upper limits of superconducting behavior. While singly crystalline substrates may be satisfactory for applications resembling semiconductor integrated circuits, their rigidity and expense preclude their use in many applications of commercial significance, for example, interconnects between computer boards, commercial power lines, and magnet windings. Therefore, flexible substrates have been investigated. Norton et al. have disclosed substrates of yttria-stabilized zirconia (YSZ) in "Y-Ba-Cu-O thin films grown on rigid and flexible polycrystalline yttria-stabilized zirconia by pulsed laser ablation," Journal of Applied Physics, volume 68, 1990, pp. 223-227. Zirconia (ZrO.sub.2) assumes a monoclinic (or tetragonal for thin films) crystal structure but is stabilized in a cubic crystal structure when combined with yttria (Y.sub.2 O.sub.3). The rigid polycrystalline YSZ substrates were stabilized with 3 mole % of Y.sub.2 O.sub.3, while the thin flexible sintered zirconia substrates were prepared using a proprietary and undisclosed sheet-forming method. When YBa.sub.2 Cu.sub.3 O.sub.7-x (YBCO) was deposited on the rigid substrates, it exhibited a T.sub.c of .about.89.degree. K. (a few degrees below the best transition temperatures for YBCO epitaxially grown on singly crystalline substrates), but when grown on the flexible zirconia substrate T.sub.c was degraded to about 85.degree. K. Norton et al, noted that the randomly oriented, polycrystalline rigid YSZ substrate produced thin films of YBCO having its c-axis highly aligned perpendicularly to the substrate. Narumi et al. have disclosed growing YBCO on a flexible metallic substrate in "Superconducting YBa.sub.2 Cu.sub.3 O.sub.6.8 films on metallic substrates using in situ laser deposition," Applied Physics Letters, volume 56, 1990, pp. 2684-2686. They used a buffer layer of 8 mole % YSZ between the YBCO and the flexible metallic substrate, which produced transition temperatures T.sub.c of no more than 83.degree. K. This further degradation in T.sub.c is considered unsatisfactory. They considered that the buffer layer reduced the effect of the oxidized metallic surface but that the buffer layer did not prevent the substrate grains from producing granular boundaries in the YBCO.